1. Field of the Invention
This invention relates to method and apparatus for controlling the operation of an electromechanical relay and more particularly to circuitry for operating an electromechanical relay to supply or remove power from a load at a preselected interval in the A.C. waveform.
2. Description of the Prior Art
In the operation of electrical household appliances, such as microwave ovens, dishwashers, and the like, electromechanical relays are utilized to connect and disconnect the load to a power source. The relays must be turned on and off at specific intervals to control the various appliance functions. It is known in the operation of microwave ovens to utilize a triac to control the flow of power from the source to the magnetron transformer. Unlike a relay, the turn on time of a triac is negligible, and little or no compensation is required in the timing of actuation of the triac to provide power to the magnetron at a specific point in the A.C. voltage waveform.
The utilization of a triac requires the incorporation of a heat sink and an optocoupler which substantially increases the cost of the solid state control circuitry. However, the utilization of a triac is preferred over a conventional electromechanical relay because of timing differences between relays and of changes which occur in the timing of the open and closure of relay contacts over the life of the relay.
If the operation of the relay is not synchronized to the voltage waveform, then the relay contacts will open or close at random points in the power line waveform If the relay is operated to open the contacts to break the load current and sufficient line voltage potential is present, a high temperature electrical arc forms between the relay contacts. Arcing causes contact erosion where the contact is destroyed, reducing the service life of the relay.
The synchronization of the relay operation with the waveform is dependent o the time interval required for closure of the relay, known as the pull in time. Due to timing variations between relays and over the life of a relay, it is not uncommon for contact breaking or closure to occur at other than the desired points on the power line waveform. For example, actuation may occur other than at the waveform crest in the switching of the magnetron transformer of a microwave oven. Consequently, when the relay contacts do not close at the desired point, such as other than the waveform crest, large current transients for an inductive load may exceed 120 amps. Voltage transients can result in arc destruction of the contacts.
In the case of a microwave oven control when the relay operation is not synchronized on the power line waveform, current transients occur, resulting in transformer vibration which customarily is recognized by an audible noise. It is the conventional practice to utilize noise suppression devices to eliminate this problem. Such devices add additional cost and complexity to the appliance control apparatus.
Initially electromechanical relays can be synchronized with the power line waveform to open and close at intervals which prevent arcing or a spark occurring between separating contacts. The synchronization is lost as the relay contacts wear and the spring bias weakens, resulting in electrical arcing between separating contacts. An electrical arc causes contact material to be eroded from one contact and deposited on the mating contact. The direction of material erosion is determined by the voltage polarity of the spark. The eroded material takes the form of small cone shaped peaks on the contacts, and eventually the contacts where the contacts may eventually stick or weld together.
Electrical arcing across relay contacts generates heat. The contact material will melt then boil as the heat becomes excessive. Material will be transferred from one contact to another during successive switching operations. Also, splattering of molten metal occurs as contacts bounce, diminishing the area of contact.
In A.C. switching the relay contacts break load current at the same approximate point on the sine wave. The same contact is always positive and the other negative at the instant of contact separation. Material is transferred from the cathode to the anode. The amount of material transferred is dependent on the severity and duration of the arc and the type of contact material used. Thus over the cycle life of a relay contact material loss can be substantial and prevent effective operation of the appliance.
One known technique for arc suppression is the positioning of a capacitor in parallel with the contacts to prevent an arc from striking as the contacts open. As the contacts open the capacitance shunts the voltage away from the contacts; however, when the contacts close again, capacitor charge is dumped on the contacts causing an arc to strike. Therefore, to prevent charge dumping a small resistance is placed in series with the capacitor. The resistance limits capacitor current but it also reduces the effectiveness of the capacitor.
In an inductive-load application it is known to use for arc suppression a clamping device, such as a varistor, in parallel with the contacts or load. In this case, when the counter electromotive force exceeds the voltage rating of the clamp, the clamp switches from a very high to very low resistance, allowing current to flow through it. If the clamping device is to be used in A.C. applications the clamp voltage must in excess of the peak of the highest possible expected rms voltage.
The known methods of arc suppression do not allow adjustments to be made in the operation of the electromechanical relay throughout the contact life. While contact arc suppression is known, there is further need to control the operation of a relay to extend the relay service life. By controlling the point at which the contacts break a load current, the life of the contacts can be significantly extended.